The present disclosure relates to an image reading device and an image forming apparatus provided therewith. More particularly, the present disclosure relates to an image reading device including an optical unit movable in the a scanning direction, and an image forming apparatus provided with such an image reading device.
Some image reading devices designed for incorporation in multifunction peripherals and the like exploiting an electrophotographic process are furnished with a document transport device that feeds sheets of a document successively onto a document stage (glass plate) to allow them to be read and that then, after completion of reading, discharges them off the document stage. Such image reading devices permit document reading by two different methods: by a sheet-through method, in which sheets of a document are read successively while being transported automatically by the document transport device with a document presser kept closed; and by a fixed-document method, in which for each sheet of a document, a document presser is opened and closed to allow it to be placed on a document stage in exchange for any previous one and a scanner unit is moved to read it. Incidentally, in the sheet-through method mentioned first, document reading proceeds while the scanner unit inside the image reading device is held at a predetermined reading position without being moved for scanning. By contrast, in the fixed-document method mentioned later, document reading proceeds while the scanner unit is moved for scanning in the sub scanning direction.
FIGS. 8 to 10 are diagrams showing an example of a conventional image reading device that permits document reading by the two methods, that is, the sheet-through and fixed-document methods. This example of a conventional image reading device is provided with, as shown in FIG. 8: a contact glass 101 comprising a hand-placed document contact glass 101a, on which a document is placed, and an automatic reading contact glass 101b, across which an automatically transported document passes; a resin member 102 provided between the two glass plates (the hand-placed document contact glass 101a and the automatic reading contact glass 101b); a scanner unit 103 which is arranged under the contact glass 101 and which includes a reading sensor (not illustrated) that reads an image on the document; a carriage 104 which holds the scanner unit 103 and which is provided so as to be reciprocally movable in the sub scanning direction (the direction indicated by arrows A and B) under the two glass plates; and a casing (frame) 105 comprising a lower frame 105a and an upper frame 105b for housing the scanner unit 103 and the carriage 104. The scanner unit 103, the carriage 104, and springs 106 (see FIG. 10) provided on the carriage 104 together constitute an optical unit 110.
Incidentally, scanner units can read images by different methods, namely a CCD sensor method employing a CCD (charge-coupled device) sensor and a CIS (contact image sensor) senor method employing a CMOS (complementary MOS) sensor.
With a CIS sensor method, the depth of field is small, and this requires the distance from the document to be maintained with high accuracy.
Accordingly, in the image reading device shown in FIG. 8, the scanner unit 103 adopting a CIS sensor method is kept in close contact with the bottom surface of the contact glass 101. Specifically, springs 106 (see FIG. 10) which bias the scanner unit 103 upward (toward the contact glass 101) are provided, and on the top surface of the scanner unit 103, sliding members 107 (see FIG. 9) which slide on the contact glass 101 while keeping contact with it are provided. This keeps a constant distance between the scanner unit 103 and the document. Incidentally, the biasing members 106 are arranged one on each of one and the other sides of the scanner unit 103 in the main scanning direction (the direction indicated by arrow C).
As shown in FIGS. 9 and 10, the image reading device is further provided with: a shaft (rail portion) 108 made of metal which supports the optical unit 110 such that this is slidable in the sub scanning direction (the direction indicated by arrows A and B); and a driving device 109 which enables the optical unit 110 to move in the sub scanning direction. The shaft 108 is arranged in a substantially central part of the optical unit 110 in the main scanning direction (longitudinal direction). The shaft 108 is abutted by an abutting portion 104a provided on the carriage 104.
The driving device 109 includes a timing belt 109a and a stepping motor (not illustrated) for driving the timing belt 109a to rotate. The timing belt 109a engages with, so as to pull, an engagement projection 104b provided on the optical unit 110 in a position lopsided to one side in the main scanning direction.
However, in the conventional image reading device shown in FIG. 8, on one hand, the optical unit 110 slides on, while keeping contact with, the shaft (rail portion) 108 arranged in a substantially central part in the main scanning direction; on the other hand, a part of the optical unit 110 lopsided to one side in the main scanning direction (that is, the engagement projection 104b) is pulled by the timing belt 109a. Thus, when the optical unit 110 is moved in the sub scanning direction, a moment is produced, causing snagging (gnawing) between the optical unit 110 and the shaft 108 or causing the optical unit 110 to vibrate. This inconveniently makes smooth scanning by the optical unit 110 difficult. Arranging the shaft 108 and the timing belt 109a in a vertical stack may prevent a moment from being produced, but that inconveniently increases the thickness of the image reading device.
Conceived to solve the problems discussed above, the present disclosure aims to provide an image reading device that allows smooth scanning by an optical unit while suppressing an increase in thickness, and an image forming apparatus provided with such an image reading device.